Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into mechanical energy and then subsequently converts the mechanical energy into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle. The rotor is coupled either directly or indirectly with a generator, which is housed inside the nacelle.
A typical modern wind turbine has many moving parts that facilitate converting the kinetic energy of the wind into electrical energy. As such, a wind turbine typically includes many bearings that provide relative movement between adjacent parts in a relatively efficient, low-friction manner. For example, in most wind turbines a “main shaft” extends from the rotor and into the nacelle and is supported by one or more “main bearings.” Additionally, the gearbox in the nacelle that steps up the angular speed of the main shaft includes several bearings. Furthermore, the yaw control system that rotates the nacelle relative to the tower to turn the rotor into/out of the wind, and the pitch control system that rotates the blades about their longitudinal axis also include various bearings that allow for enhanced operation of the wind turbine.
Conventionally, these bearings are configured as roller element bearings characterized by having a structural element (e.g., a ball bearing) disposed between the two components which are moving relative to one another. Roller element bearings fail for any number of reasons, but ultimately their life is limited by surface fatigue and wear. Such limited-life components require regular maintenance so as to avoid larger scale failure modes. The replacement parts and maintenance for such limited-life components increase the overall costs of operating a wind turbine. Accordingly, wind turbine and bearing manufacturers strive for improved or alternate designs that extend the operating life of the bearings.
Plain bearings are generally known in the art as having a long operating life. The main reason for this is that, unlike roller element bearings, plain bearings do not have any structural element disposed between the two relative moving components, but instead have only a fluid film disposed therebetween. Thus, the wear and fatigue issues associated with roller elements, as well as the costs associated with their replacement and maintenance, may be avoided. Consequently, plain bearings appear to provide an attractive alternative to roller element bearings. Additionally, plain bearings are designed to eliminate surface-to-surface contact and thus operate with even lower friction, which may further increase efficiency.
There are two primary types of plain bearings for supporting shafts: hydrostatic bearings and hydrodynamic bearings, each typically having a rigid housing with an inner surface opening (the bearing) closely fitted around the shaft (the journal) and a fluid film between the journal and the bearing. In a hydrodynamic bearing, the rotation of the journal self-pressurizes the fluid film in a wedge between confronting journal and bearing surfaces so as to support the load and maintain the journal separate from the bearing. Hydrostatic bearings, on the other hand, include an external pump that pressurizes the fluid film around the journal (independent of the shaft dynamics) to support the load and maintain the journal separate from the bearing. These bearings, while having certain desirable attributes, also have certain drawbacks that have made their implementation within the wind turbine industry rather limited.
In this regard, and in reference to hydrodynamic bearings, unless the journal is rotating with sufficient speed, the fluid film may not be able to fully support the load and maintain the journal separate from the bearing. In this case, the hydrodynamic bearing does not operate in a full-film condition, but instead operates in a boundary condition, wherein the load is partially carried by the fluid film and partially carried by direct surface contact with the bearing. Operating a hydrodynamic bearing in a boundary condition can cause wear or damage that may significantly shorten the operating life of the bearing.
Hydrostatic bearings, on the other hand, are capable of supporting the load even when the journal is rotating slowly or not at all. However, to effectuate external pressurization of the fluid film, hydrostatic bearings include a number of pockets or cavities typically formed in the bearing surface which are supplied with lubricating fluid (e.g., oil, grease, etc.) from an external reservoir and pressurized by an external pump. Hydrostatic bearings, while ensuring sufficient pressurization to support the load and provide separation of the journal and bearing, have reduced operating efficiency as compared to, for example, hydrodynamic bearings. In this regard, the pockets formed in the bearing surface create drag or otherwise affect the film hydrodynamics in a negative manner. This effect becomes pronounced when the journal is rotating at a relatively high speed which would otherwise support the load by the hydrodynamics alone.
Wind turbines rely on the wind and consequently, have a wide operating range, from being in a stand still mode (no rotation of the rotor, main shaft, gears, etc.), to operating at relatively low angular velocities of the various shafts and mechanisms under, for example, low wind conditions, and to operating at relatively high angular velocities under, for example, high wind conditions. Due to the unpredictability of the wind, start-up and shut-downs that occur with wind turbines, and the resultant range of operating conditions, manufacturers have traditionally relied on roller element bearings in wind turbine designs.
While roller element bearings are adequate for their intended purpose, manufacturers continually strive to improve the design, operating costs, and functionality of wind turbines. To this end, it would be desirable to utilize plain bearings instead of roller element bearings in wind turbine designs to increase the operating life of the bearings and decrease the costs associated with replacement and maintenance of roller element bearings.